Automatic volume control



May 14,` 1946.

M. cAwElN AUTOMATIC VOLUME CONTROL 'Filed Sept. 18, `1.945

ATTORNEY Patented May 14, 1946 AUTOMATIC VOLUIHE CONTROL Madison Cawein, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation,

a corporation of Delaware Application September 18, 1943, Serial No. 502,863

Claims.

This invention relates to radiant energy. receivers and particularly to receivers employed for simultaneously receiving signals from a multiplicity of channels.

According to conventional practice in the operation of radiant energy receiving devices, it always is desirable and in many cases is essential to control the amplification of the received signais by applying a control voltage to the initial stages of the receiver in order to prevent the overloading of' subsequent stages. This is particularly true where the receiver is to be operated in a strong field.

In the operation of multichannel receiving devices it frequently is necessary to utilize an automatic means for controlling the gain of the amplifying devices in order to maintain the amplifled signal levels substantially constant. In the case of a television receiver for example, where a single device is employed to receive and amplify both the video and audio signals, the initial stages of the receiver amplify both types of signals. 'I'he video signals are modulated upon a carrier wave of one frequency while the audio signals are modulated upOn a carrier wave of a different frequency. Usually, both of the carrier frequencies are simultaneously amplified by a suitable radio frequency amplifier and converted into two intermediate frequencies which also are simultaneously amplified by a single device of suitable design. Subsequently, the two intermel diate frequencies are separated and are subjected to individual amplification and demodulation for application to respective video and audio reproducing devices. f

Heretofore a control voltage representative of the levelof one of the signals has been developed and utilized to control the gain of the v-arious amplifying devices, including those'which simultaneously amplify both video and audio signals. For example, a control voltage representative of the video signal level has been used to regulate the -gain of the radio frequency and intermediate frequency amplifiers which pass both video and audio signals. In some cases the amplifier gain has been controlled bythe voltage representative of the audio signal level.

As is well understood in the art, however, it frequently occurs that the levels of the video and audio signals do not change proportionally or even in the same sense. Such a condition may arise from any one or a combination of numerous circumstances or conditions. For instance, in the vicinity of the receiver there may be located ,a structure of such a character that substantial amounts of the audio signal carrier frequency energy are absorbed without materially affecting the video signal carrier frequency energy, If the automatic volume control of the receiver is effected by means of a voltage developed from the video signal, it is evident that such a voltage will not control the amplifier gain in a manner to produce an increase in the audio signal level to compensate for the energy absorption thereof. Such an automatic volume control system is additionally deficient if there is an increase in the video signal level effected without a corresponding increase in the' audio signal level. In such a case the control voltage responsive to the n-Y creased video signal level is of such a character to reduce the amplifier gain. This, of course, has the effect of reducing the amplified signal level of the audio signals still further.

It is an object of the present invention, therefore, to provide an automatic volume control arrangement whereby there is effected an equalization of the signal levels in a multichannel receiver where large discrepancies exist in the carrier signal levels of the respective channels.

Another object of the invention is to provide, in a television receiver for both audio and video signals, an automatic volume control arrangement whereby the gain of amplifiers employed to amplify both types of signals is controlled in a manner which is not discriminatory of either type of signal.

A further object of the invention is to provide a novel method of automatically controlling the gain of multichannel amplifiers whereby to obviate discrimination among the signal levels of the channels.

In accordance with the instant invention there is provided individual means for developing control voltage representative, respectively, of the individual signal levels of a plurality of signaling channels. the signals from which are susceptiblegain control facilities of its associated subsequent stages. v

In accordance with the illustrative embodiment of the invention disclosed herein, a television receiver for both video and audio signals comprises the multichannel receiver. A control voltage representative of the video signal level is developed and impressed in fulll magnitude upon the individual video signal amplier stages. Likewise, there is developed a control voltage representative of the audio signal level which is impressed in full magnitude upon the individual audio signal amplifier stages. Predetermined portions of these control voltages-are combined to form a composite control voltage which is impressed upon the radio frequency amplier and other stages of the receiver which are designed to pass both the video and audio signals.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing, and its scope will be pointedout in the appended claims.

In the accompanying drawing the single gure is a circuit diagram, partly schematic, of a television receiver embodying the instant invention.

Referring now to the drawing, the circuit diagram is that of a television receiver capable of receiving video and audio signals which are modulated, respectively, on carrier waves of two different frequencies. These signal modulated carrier waves comprise a multichannel signaling system. The radio 'frequency signals of both channels are collected by an antenna I which is connected to a wide band radio frequency amplifier 2. The radio frequency signals of both channels are converted, respectively, into audio and video intermediate frequencies by means of a frequency converter 3. The two intermediate frequencies are amplied by a. wide band intermediate amplifier 4. The band widths of the radio frequency and intermediate frequency amplifiers 2 and 4,

respectively, and the frequency converter 3 are suillcient to pass both of the signal carrier waves. The output circuit of the combined audio-video intermediate frequency amplier 4 is connected to two separate channels. The video signal channel comprises a number of intermediate frequency ampliers such as 5 and E, which are designed to pass only the video intermediate frequency. The amplifier 6 is connected to a video signal detector I by means of which the video signals are derived from the intermediate frequency carrier wave. The video signals subsequently are amplified by an amplifier 8 which is connected to the detector 'I; and the amplied video signals are impressed upon a visual reproducing device 9. In addition to the video signal amplification and demodulation in this channel of the receiver, the synchronizing and blanking signals are derived from the carrier wave and separated from the video signals by conventional means (not shown). These system control signals are utilized to control the scanning frequency generators and similar apparatus in a well known manner.

The other of the two receiver channels is for the purpose of deriving from the carrier wave the audio signals and translating them for use to control a suitable reproducing device. This channel comprises a number of audio intermediate frequency ampliers such as I0 and II, the latter of which is connected to an audio detector I2. The detector functions to demodulate the audio intermediate frequency carrier wave to derive from the carrier wave the audio signals which subsequently are amplified by an audio signal amphner Il connected to the output circuit 'of the detector. The amplified audio signals are impressed upon an aural reproducing device III such as a conventional loud speaker.

Associated with the video signal channel there is provided an AVC rectifier I5 connected to the output circuit of the video intermediate frequency amplifier 6. The rectier I5 may comprise a conventionally connected diode by means of which the video signals are recovered from the intermediate frequency carrier wave. 'I'he output circuit of the rectifier I5 includes a load resistor I6 across which there is developed a unidirectional voltage representative of the video signal level. The load resistor I6 is shunted by a condenser Il in the usual manner, whereby the voltage developed across the load resistor varies in accordance with the envelope of the video intermediate frequency carrier wave.

There also is connected in shunt with the output circuit of the rectifier I5 the usual AVC lter, comprising a resistor I8 and a condenser I9. The full voltage developed across the rectifier load resistor I6 is impressed by means of a conductor 20 upon the input circuit of the video intermediate frequency amplifier 5 to constitute a bias, whereby to control automatically the gain of this ampllifier in accordance with the video signal eve Similarly, there is provided for the audio receiver channel an AVC rectifier 2l which is connected to the output circuit of the audio intermediate frequency amplifier II. This rectifier, likewise, may be a conventionally connected diode, in the output circuit of which there is connected a load resistor 22 shunted by a condenser 23. The

voltage developed across the load resistor varies in accordance with the envelope of the audio intermediate frequency carrier wave. The full voltage developed across the load resistor is connected through an AVC filter comprising a resistor 24 and a condenser 25 and by way of a conductor 26 to the input circuit of the audio intermediate frequency amplifier I0. This voltage is utilized as a bias for the amplifier I0 whereby to control automatically the gain thereof in accordance with the audio signal level.

In addition to the described automatic volume control connections to the individual video and audio intermediate frequency amplifiers 5 and I0, respectively, there are provided other connections from the video and audio AVC rectifiers I5 and ZI, respectively. .A connection across the load resistor I6 of the video rectifier I5 includes a potentiometer 21 and by-pass condenser 28. The circuit arrangement of these two elements is essentially a filter arrangement similar. to that comprising the resistor I8 and the condenser I9. In this case, however, the potentiometer provides a means for utilizing any predetermined portion of the voltage developed across the load resistor IIi.A A similar filter arrangement Comprising a potentiometer 29 and the condenser 28 is connected across the load resistor 22 of the audio AVC rectifier 2i. In this case also, the potentiometer adjustment permits the utilization of any predetermined portion of the voltage developed across the load resistor 22.

The two potentiometers 21 and 29 are connected together and, by means of a conductor 30, furnish a ycomposite control voltage to the input circuits of the radio frequency amplifier 2, the' frequency converter 3' and the audio-video intermediate frequency amplifier 4. The `composite control voltage functions to adjust automatically the gain of these devices. If desired, the two potentiometers 21 and 29 may be mechanically connected, as shown by the broken line, whereby simultaneous adjustments thereof may be made by means of a control knob 3|. The interlinkage of the potentiometers preferably should be one which will provide for an increase in the voltage derived from one of the AVC rectifier load resistors concurrently with. a proportionate decrease in the voltage derived from the other AVC rectifier load resistor. A

Referring now to the operation of the automatic volume control system in accordance with the instant invention, assume that the level of the video signal received by the antenna I is normal and the level of the audio signal received by the antenna is subnormal. Neglecting for the moment a consideration of the concurrent operations of the twointerconnected automatic vol- 'ume control circuits, it is evident that there would be developed across the load resistor IB a normal AVC voltage which is representative of the normal signal level of the received video signals. If this voltage were permitted to function independently to control the various amplifiers to which it is supplied as a bias, the gain of these amplifiers would be regulated to maintain the normal level of the video signals. v

Now consider the effect of the audio AVC systern. If it were permitted to function independently of the video AVC system, there would be developed across the load resistor 22 a control voltage representative of the subnormal level oi' the audio signals. rlihis voltage, serving as the sole automatic volume control voltage of the various amplifiers to which it is furnished, would then change the 'bias of the amplifiers in a manner to increase the gain thereof. 'I'he tendency would be to increase the amplifier gain sufficiently to impress anaudio signal of normal level upon the reproducing device I4.

However, in accordance with the invention, the initial amplifiers of the receiver are subject to the joint control of the video and audio AVC system. Assume that the potentiometers 21 and 29 are adjusted to employ one-half of the video AVC voltage developed across the load resistor I6 and also one-half of the audio AVC voltage developed across the load resistor 22. It is obvious that the composite AVC voltage impressed upon4 only one-half of the audio AVC voltage is impressed upon the common stages, the' increase in the gain thereof is not suiiicient to compensate for the reduced signal level of the received audio signals. However, lthere will be-an increase in the amplification of both video and audio signals. In the video channel of the receiver the increased level of the video signals will tend to alter the video AVC voltage in a manner tending to restore the level of the video signals to normal. At the same time, since there has been some' increase in the audio signal level effectedin'the common signal stages, the audio AVC voltage likewise will be altered in a sense opposite to the change in the video AVC Voltage.

It is apparent that there will be developed a composite AVC yvoltage for impression upon the common signal stages which is a compromisebetween the voltage which would be required to bias these stages for the production of normal `than normal. In response to this condition there is developed by the video AVC system a control voltage of a character to reduce the video signal level to normal. The entire magnitude of this voltage is applied to control the gain of the video intermediate frequency amplifier 5. The result of this arrangement is the production of a normal level video signal suitable for impression upon the visual reproducing device 9. y

By reason of the compromise gain control of the common signal stages by the composite AVC voltage, the level ofthe audio intermediate frequency signals still is somewhat below normal. Responsive to this condition, the audio AVC system develops a voltage across the load resistor 22, the entire magnitude of .which is impressed upon the audio intermediate frequency amplifier I0. The gain of this amplifier is increased thereby sufficiently to produce a normal level audio signal suitable for impression upon the aural reproducing device I4.

In order to effect the functioning of the automatic gain control system in accordance with this invention it is necessary to employ resistance elements which are interrelated in valuesaccording to a particular plan. It will be obvious to those skilled in the art that a. considerable range of values for the resistors may be employed so long as they are properly related. `In general, this relationship may be expressed by letting R represent the value of resistors I8 and 24, mR represent the value of resistors I8 and 22 and nR represent the value of potentiometers 21 and 29. A system embodying the invention may be successfully operated so'long as the ratio m/Zn is kept small, of the order of 10%. l

As a' specic example of the general relationship of the values of the various resistance elements, the following values are given, it being understood that resistance elements having other values may be employed with equal facility lwithout departing lfrom the spirit of the invention so long as these other values adhere to the general relationship specified in the preceding paragraph:

Resistors I6 and 22 megohms-- 0.5 Condensers I1 and 23 microfarads-- 0.1 Resistors I8 and 24 megohms-- 5.0 Condensers I9, 25 and 28 microfarads 0.005 Potentiometers 21 and 29 megphms-- 2.0

The foregoing description of the operation of the illustrative embodiment of the invention has been given on th`e assumption that one-half of the AVC voltages developed by the respective video and audio systems is utilized to form the composite AVC voltage. It is contemplated that, by

means of the described adjustments of the poi signaling channels in the manner disclosed'herein, whereby the advantages to be derived may also be secured in a multichannel system comprising any number of channels.

To those skilled in the art it'will be apparentv that the instant invention provides for the securing of numerous advantages over automatic volume control arrangements-heretofore employed.

The most important advantage is that large discrepancies in the levels of the intelligence signals on the various channels tend to be equalized. Such an advantageous result is particularly-important in the case of a television receiver such as that described herein. The adjustments of the potentiometers 21 and 29 may be made for each individual receiver at the time of its installation by an installer who is skilled in making such adjustments. Once made, there generally will be no occasion for changing these adjustments and the receiver may be operated in a, normal manner regardless of any discrimination made between the levels of the audio and video signals by reason of local conditions in the vicinity of the receiver.

While there has been described what, at present is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:` A

1. In a multi-channel signal receiver, a common amplifier forall of said channels, an individual amplifier for each of said channels, means for developing a voltage representative of signal level for each of saidV channels, means for impressing each of said developed voltages upon the input circuits of said respective individual signal amplifiers to automatically control the gain thereof, and means for impressing portions of each of said developed voltages upon the input circuit of said common amplifier to automatically control the gain thereof.

2. In a signal receiver for receiving signals of different types, an amplifier to amplify all of said signals, means for developing a first unidirectional voltage representative of a first signal level, means for developing a second unidirectional voltage representative of a second signal level,-

means for deriving a composite unidirectional voltage comprising predetermined percentages of said first and second unidirectional voltages, and means for impressing said composite unidirectional voltage upon said amplifier to automati" cally control the amplification thereof.

3. In a signal receiver for receiving signals of different types, radio frequency and intermediate frequency amplifiers to amplify all of said signals, means for developing a first unidirectional voltage representative of the signal level of one type of signal, means for developing a second unidirectional voltage representative of the signal level of another type of signal, means for deriving a composite unidirectional voltage comprising predetermined percentages of said first and second unidirectional voltages, and means for impressing said composite unidirectional voltage upon said radio frequency and intermediate frequency amplifiers to automatically control the amplification thereof.

4. In a signal receiver for receiving signals of two different types, an amplifier to amplify simultaneously received signals of both of said types, an amplifier to amplify only signals of a first one of said types, an amplifier to amplify only signals of a second one of said types, means for developing a, rst unidirectional voltage representative of the signal level of said first type of signal, means for developing a second unidirectional voltage representative of the signal level of said second type of signal, means for impressing said first unidirectional voltage upon the input circuit of said first type signal amplifier to automaticalLv control the gain thereof, means for impressing said second unidirectional voltage upon the input circuit of said second type signal amplifier to automatically control the gain thereof, and means for impressing portions of said first and second unidirectional voltages upon the input circuit of said first named amplifier to automatically control the gain thereof in accordance with a predetermined relationship between the signal levels of said first and second types of signals.

l5. In a signal receiver for receiving two different types of signals, radio frequency and intermediate frequency amplifiers to amplify both types of said signals, an intermediate frequency amplifier to amplify only signals of a first one of saidtypes, an intermediate frequency amplifier to amplify only signals of a second one' of said types, means for developing a first unidirectional voltage representative of the signal level of said first type Ofsignal, means for developing a second unidirectional voltage representative of the signal level of said second type of signal, a voltage divider associated with each of said voltage developing means, means including said voltage dividers for deriving a composite unidirectional voltage comprising predetermined percentages of said first and second unidirectional voltages, means for impressing said composite unidirectional voltage upon said radio frequency and intermediate frequency amplifiers common to both types of signals, and means for impressing said first and second unidirectional voltages upon said first and second type signalintermediate frequency amplifiers, respectively, to automatically control the amplification thereof.

6. In a television receiver for both video and audio signals, radio frequency and intermediate frequency amplifiers to amplify both types of said signals, an intermedia frequency amplier to amplify only said video signals, an intermediate frequency amplifier to amplify only said audio signals, means for developing a unidirectional voltage representative of the video signal level, means for developing a unidirectional voltage representative of the audio signal level, two potentiometers, each associated with respective ones of said voltage developing means, means including said potentiometers for deriving a composite unidirectional voltage comprising predetermined percentages of said video and audio signal level representative voltages, means for impressing said composite voltage upon said radio frequency and intermediate frequency amplifiers, and means for impressing said video and audio unidirectional voltages upon said video and audio intermediate frequency amplifiers, respectively, to automatically control the amplification thereof.

'7. In a television receiver for both video and audio signals, radio frequency and intermediate frequency amplifiers to amplify both types of said signals, an intermediate frequency amplifier to amplify only said video signals, an intermediate frequency amplifier to amplify only said audio signals, a rectifier for developing a unidirectional voltage representative of the video signal level,

' a rectifier for developing a unidirectional voltage aecomo 5 resentative voltages, means for impressing said composite voltage upon said radio frequency and intermediate frequency ampliners. and means for impressing said video and audio unidirectional voltages upon said video and audio intermediate frequency amplifiers, respectively, to automatically control the amplincatlon thereof.`

8. In a multichannel signal receiver, signal amplifying means common to all of said channels.

means for developing individual voltages reprefor combining predetermined portions of said respective developed` voltages to produce a composite voltage, and means for impressing said composite voltage upon said amplier to control automatically the ampliilcation thereof.

10. In a multichannel signal receiver, an ampliiler for amplifying signals of all ot said channels. means for deriving from said amplifier individual signals from said respective channels, means for developing from each of said derived'signals a f voltage representative of the respective levels of said signals, means for combining predetermined portions oi said respective developed voltages for producing a composite voltage, and means for impressing said composite voltage upon said amplier to control automatically the ampliilcation thereof.

' MADISON CAWEIN. 

